Effects of heavy metals on the adsorption of ciprofloxacin on polyethylene microplastics: Mechanism and toxicity evaluation

Microplastics are plastic particles less than 5 mm in diameter and are widely present in water environments. Their unique surface structures can adsorb coexisting pollutants in the surrounding environment, such as antibiotics and metal ions, leading to compound pollution. The adsorption of ciprofloxacin on polyethylene microplastics under different environmental conditions (pH and salinity) was investigated. The Freundlich model fitted well at 25 °C, indicating that the adsorption of ciprofloxacin by polyethylene microplastics was multilayered, and Fourier Transform infrared spectroscopy (FTIR) analysis indicated that the adsorption of ciprofloxacin by polyethylene microplastics was physical. The kinetic adsorption of ciprofloxacin on polyethylene microplastics followed a pseudo-second-order mode. Heavy metals (Cu2+, Cr3+, Cr6+, Cd2+, and Pb2+) affected the adsorption of ciprofloxacin by microplastics, which was related to the type and concentration of metal ions and the valence state of the ions. The acute toxicity of microplastics and the microplastic-ciprofloxacin-Cu2+ complex were evaluated using luminescent Photobacterium phosphoreum, demonstrating the polyethylene toxicity microplastic-ciprofloxacin-Cu2+ complex was mainly caused by Cu2+ and ciprofloxacin rather than microplastics. This study provides theoretical support for the environmental behavior and ecological effects of microplastics in aqueous environments.